The present invention relates to a gas permeable probe for use in an optical analyzer for an exhaust gas stream flowing through a duct or chimney, the probe comprising:
an elongate hollow structure having first and second ends and a side wall, with an optical cavity defined between the first and second ends within the side wall,
a mounting structure at the first end and adapted for mounting the elongate hollow structure within the duct or chimney,
a support member at the second end,
a connecting structure connecting the mounting structure at the first end to the support member at the second end,
an optical window at the first end permitting a beam of light originating from an optical analyzer to enter into the optical cavity to travel from the first end to the second end,
a filter forming a part of the side wall, and
a retroreflector provided at the second end for returning the light beam to the first end of the hollow structure.
A gas permeable probe of this kind is known, for example, from U.S. Pat. No. 4,560,873. The gas permeable probe disclosed in this reference utilizes a cylindrical ceramic filter to permit gas flowing through the chimney to enter into the optical cavity, with the pores of the filter being sized such that particulate material in the chimney is prevented from entering the optical cavity. A similar gas permeable probe is disclosed in U.S. Pat. No. 6,064,488 in which the elongate hollow structure comprises a tube having slots relieved in the upper and lower surfaces thereof with filters of sintered metal being welded into the windows to allow gas flowing through a chimney to enter into the optical cavity. The porosity, area and location of the filters in the known arrangements determine the rate at which gas diffuses through the optical cavity. Gas permeable probes of the above kind are used in optical analyzers designed to carry out spectral analysis of gases contained in the optical cavity. Since the gases contained in the optical cavity correspond to the gases flowing through the duct or chimney it is possible, using spectral analysis, to obtain information on the types of gas that are present in the duct or chimney and their relative concentrations.
Moreover, a gas permeable probe of this kind can also be used to obtain information on various types of dust and dust contents in gas flows such as exhaust streams. This can be done if the pore size of the filter is selected such that the dust of interest can enter into and escape from the optical cavity.
Gas permeable probes of the kind to which the present application relates can be used in gas carrying ducts, especially exhaust ducts of all kinds which operate in a temperature range of e.g. 50xc2x0 C. to 450xc2x0 C. Such ducts are, for example, found in power stations, refuse burning plants, in cement works, in association with large furnaces, in steelworks and in gasworks.
While known gas permeable probes of the initially named kind are suitable for certain applications, they all suffer from various restrictions, so that it is difficult to use one basic type of apparatus for a variety of different measurements and applications. For example, different applications require different types of filters with different characteristics, such as pore size and hydrophobic characteristics.
Furthermore, it is frequently necessary, depending on the type of measurement that has to be carried out, to use optical windows and retroreflectors of different materials, i.e. of materials having different optical characteristics. This is however not readily possible with gas permeable probes of the known kind.
Accordingly, it is an object of the present invention to provide a gas permeable probe in which the optical window and/or the retroreflector can be readily interchanged or replaced without experiencing difficulties with the alignment of the retroreflector relative to the optical window.
It is a further object of the present invention to provide a type of modular design of a gas permeable probe which can be adapted in a relatively simple manner for use in a wide range of applications with respect to the gas temperature, gas pressure, water content, gas concentration, type of gas and with respect to the most diverse types of dust and dust contents.
At the same time, the gas permeable probe should operate reliably over a long period of time in a simple manner with a low servicing requirement and should be capable of being reliably calibrated with respect to the gases being detected while eliminating sources of error.
In order to satisfy these objects there is provided a gas permeable probe for use in an optical analyzer for an exhaust gas stream flowing through a duct or chimney, the probe comprising:
an elongate hollow structure having first and second ends and a side wall, with an optical cavity defined between the first and second ends within the side wall,
a mounting structure at the first end and adapted for mounting the elongate hollow structure within the duct or chimney,
a support member at the second end,
a connecting structure connecting the mounting structure at the first end to the support member at the second end,
an optical window at the first end permitting a beam of light originating from an optical analyzer to enter into the optical cavity to travel from the first end to the second end,
a filter forming a part of the side wall, and
a retroreflector provided at the second end for returning the light beam to the first end of the hollow structure,
the optical window being releasably mounted at the first end of the elongate hollow structure and/or the retroreflector being releasably mounted at the second end of the elongate hollow structure.
This arrangement makes it relatively easy to change the optical window and/or the retroreflector to enable adaptation of a basic gas permeable probe to different applications. Moreover, the connecting structure connecting the mounting structure at the first end to the support member for the retroreflector at the second end ensures that the retroreflector is always correctly aligned with the optical window provided at the first end.
The retroreflector is preferably releasably connected to the support member at a side of the support member remote from the optical window and aligned with an opening in the support member. This makes it possible to remove and replace the retroreflector without having to dismantle anything other than the structure readily accessible in the immediate vicinity of the retroreflector at the second end of the elongate hollow structure.
Thus, the design makes it possible to readily exchange the retroreflector, which is mounted on the support member without it being necessary to disturb the filter.
The gas permeable probe preferably further comprises a ring recess having a base and formed in the mounting structure at the first end of the elongate hollow structure, with the optical window being disposed in the ring recess and being accessible when a filter forming part of the elongate hollow structure is removed.
Thus, the optical window can be readily exchanged after removal of the filter.
The mounting structure preferably comprises a first mounting flange at the first end of the elongate hollow structure. This first mounting flange provides a simple way of releasably mounting the optical window and the filter structure in the gas permeable probe.
In a particularly preferred arrangement the gas permeable probe further comprises a support tube connected to the first mounting flange and extending to a second mounting flange adapted for mounting to a wall of the duct or chimney. This enables the optical cavity to be mounted within the duct or chimney away from the wall of the duct or chimney, and thus in a position in which it is fully exposed to the flow through the duct or chimney, without the measurement being disadvantageously affected by boundary layer wall effects of the duct or chimney.
The ring recess is conveniently provided in the first mounting flange.
In a preferred embodiment the first mounting flange has a first side adjacent the elongate hollow structure and a second side remote from it and a pressure ring is provided at the first side. The pressure ring conveniently has a ring-shaped axial projection engaging into the ring recess in the first mounting flange.
First and second ring seals are expediently provided, with the first ring seal being disposed between the optical window and the base of the ring recess and the second seal being provided between the optical window and the axial projection of the pressure ring. This arrangement ensures satisfactory sealing at the optical window while avoiding mechanical stress in this component and thus possible damage thereto.
A plurality of threaded fasteners which extend through the pressure ring and the mounting flange are conveniently used for clamping the first mounting flange and the pressure ring together. The threaded fasteners usefully engage into a ring-shaped connecting member provided at a side of the first mounting flange remote from the pressure ring.
When the filter includes a connection flange disposed adjacent the first end of the elongate hollow structure, the threaded fasteners conveniently also pass through the connection flange. The threaded fasteners preferably engage into a ring-shaped connecting member provided at a side of the mounting flange remote from the pressure ring and the ring-shaped connecting member conveniently has an axial projection which supports a ring-shaped heater mounted thereon.
The filter preferably comprises an elongate modular filter forming part of the elongate hollow structure. The modular filter expediently has first and second opposite ends and includes a filter structure having at least one filter member, a bellows at one of the first and second opposite ends adjacent the filter structure, the connection flange at the first opposite end and a further connection flange at the second opposite end adjacent the support member. In this way the pressure ring and the optical window are removable on releasing the threaded fasteners and removing the elongate modular filter.
Although it is considered preferable to use a filter structure comprising a tube of filter material, it is also possible to use a filter structure comprising a metal tube having windows therein which are occupied by elements of filter material.
Further advantages of the invention will be set forth in the subsequent description given by way of example only with reference to the preferred embodiment as illustrated in the drawings.